1. Field of the Invention
This invention relates to the control of hydraulically powered equipment and more particularly it concerns novel means for improving the precision of actuation of such equipment.
2. Description of the Prior Art
The present invention is particularly useful in the control of operation of hydraulic pile driving hammers. These hammers employ hydraulic piston and cylinder means to move a heavy ram up and down so that it hammers on an anvil which rests on a pile or other element to be driven. Because of the inertia of the heavy ram, it does not respond instantaneously to a reversal of forces in the hydraulic piston and cylinder means which moves the ram. The actual switching of hydraulic forces in the piston and cylinder means must take place before the ram reaches at least the upper end of its hammering stroke. Moreover, because of variations in hydraulic pressures, ground conditions and other factors, the timing of hydraulic switching should be variable in relation to ram position. That is, under some conditions the hydraulic switching should take place when the ram is at one position, while under other conditions the switching should take place when the ram is at a different position.
It has been proposed to effect hydraulic switching in hammering systems by use of pilot ports which open into the hydraulic piston and cylinder means at locations such that when the ram reaches a predetermined position, the ports become exposed to sudden changes in hydraulic pressure. These pressure changes, and their accompanying fluid flow, pass through pilot conduits to the main hydraulic valve of the system, normally referred to as the cycling valve. The pressurized fluid from the pilot system serves to actuate the valve and thereby switch or reverse the hydraulic forces which control ram movement.
The use of a pilot conduit which becomes exposed to hydraulic pressure only when the hammer ram reaches a predetermined position raises certain problems when the switching action is to be adjusted with respect to ram position. The flow of fluid through the pilot conduit system can, of course, be adjustably metered so as to delay, more or less, the complete actuation of the main hydraulic valve. Such metering, however, causes the valve actuation to take place quite slowly with a resultant loss of precision.